pfsense_openvpn_client support for auth_user and auth_pass fields

[samuel-emrys]

Is your feature request related to a problem? Please describe.
When configuring a vpn client, there currently doesn't appear to be a way to populate the "Username" and "Password" fields of the "User Authentication" section. In the xml, these are the following:

<openvpn>
  <openvpn-client>
    <auth_user></auth_user>
    <auth_pass></auth_pass>
  </openvpn-client>
</openvpn>

This is useful when trying to configure an external VPN service, such as PIA.

Describe the solution you'd like
Add the fields auth_user and auth_pass to the api to populate these fields in the xml.

Describe alternatives you've considered
N/A

Additional context
Add any other context or screenshots about the feature request here.

[opoplawski]

Would you be up for filing a PR for this? It should be fairly straightforward.